In the paper-making process for newspaper and telephone directory paper and so, various yield-improving systems have been adopted for the purpose of improvement in the yield of fine fibers and fillers and improvement of freeness. Among them, the technology using polyethylene oxide as a yield-improving agent has an advantage in that it is not affected by a large amount of water-soluble anionic substance and suspended colloid substance contained in the paper stuff. In the yield-improving system by PEO, it is not substantially used alone but various agents are used in combination (yield-improvement promotor). Various water-soluble phenol resins have been developed as effective promotors.
However, when a well-known water-soluble phenol resin is added, the pH of paper stuff becomes high. When the amount of the phenol resin is increased, the paper product is discolored, the life of the product is reduced and it is hardened by thickening and self-crosslinking.
Further, in the case of a combination of a phenol resin and PEO according to the conventional technology, it has been pointed out that the yield becomes unstable by admixture of a deinking agent originated from deinked pulp (referred to as DIP hereinafter) thereby effecting the yield of fillers and making the removal of sticky materials are insufficient.
The subject of the present invention is to solve the conventional problems described above and to provide a method for the improvement of yield and freeness in the paper-making and waste water treating process which gives no discoloration of paper product and extends the product life and can exert stable yield improving effect on the paper product containing a large amount of DIP and filler.
Further, the subject of the present invention is to provide an excellent yield improvement promotor for paper-making which is a phenol resin used in combination with PEO in the method and gives no discolorarion of paper product and extends the product life and can exert stable yield improving effect on the paper product containing a large amount of DIP and filler.
We, inventors, have investigated eagerly to solve the above problems and have found that a reaction product able to provide an aqueous solution of neutral to acidic, to give no discoloration of paper product and to extend the product life and to exert stable yield improving effect on the paper product containing a large amount of DIP and filler can be obtained by sulfonating a phenol and then condensing it with formaldehyde, and have found that the improvement of yield and freeness in the paper-making and waste water treating process can be attained when the reaction product is used in combination with PEO to complete the present invention.